This invention relates to carbon brushes, and in particular, a method for attaching a shunt wire to a brush body, an improved cement for attaching a shunt wire to a brush body, and the brush obtained using this cement.
In the production of carbon brushes for electrical motors, it is well known to attach a shunt wire to a brush body by tamping. In this process, known for example from U.S. Pat. No. 4,579,611, a cement mixture containing a conductive powder and a thermosetting resin is tamped around a shunt wire. Subsequently, the resin may be cured by heating or by a chemical reaction. The resin enhances the stability and integrity of the tamping connection. Presently, these resins are mostly phenolic resins. While polyimides have also been used, they are undesirable from an environmental point of view.
Tamped brushes that are used in fuel pumps, particularly for automotive applications, are often exposed to the fuel that the pump is conveying. As more cars are being fueled with gasoline that contains various additives and alcohol, the long term stability of the tamping cement is no longer guaranteed, as the alcohol and additives tend to cause deterioration of the resin in the cement. This deterioration manifests itself in increased resistance between the shunt wire and the brush.
It is also known to produce tamped brushed from silver-plated copper powder without resin. Such brushes are extremely stable in alcohol-based fuels, but the pull out force is relatively low, especially in shallow tamp holes, as these brushes rely solely on mechanical deformations and stress forces in the copper powder to keep the shunt secured in place.